CVE-2026-21388 Unbounded Request Body Read in MS Teams Plugin {{/lifecycle}} Webhook Endpoint

What is CVE-2026-21388?

CVE-2026-21388 is a low-severity denial of service vulnerability in Mattermost Mattermost Server, published on 9 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Mattermost Plugins versions <=2.3.1 fail to limit the request body size on the {{/lifecycle}} webhook endpoint which allows an authenticated attacker to cause memory exhaustion and denial of service via sending an oversized JSON payload. Mattermost Advisory ID: MMSA-2026-00610

Weakness: CWE-770

Which versions are affected by CVE-2026-21388?

3 open source packages ship an affected version, including github.com/mattermost/mattermost-plugin-msteams. It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2026-21388?

Upgrade github.com/mattermost/mattermost-plugin-msteams to 1.15.1-0.20260213190728-6fe4d295592e or later.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-21388 being exploited?

There is no known exploitation of CVE-2026-21388 in the wild and no public exploit code.

Other vulnerabilities in Mattermost Mattermost Server

• CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers Critical 9.9
• CVE-2025-12421 Account Takeover via Code Exchange Endpoint Critical 9.9
• CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User Critical 9.9
• CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards Critical 9.9
• CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive Critical 9.9

All Mattermost Mattermost Server vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-x274-8qfc-hrgf, GO-2026-5734.